1. Field of the Invention
The present invention relates to a mobile communication transmitter mounted on a mobile station for communicating with a base station.
2. Description of the Related Art
In a portable telephone set constructed by a transmitter for mobile communication, amplifiers constituting a transmission power amplifying section take most of the power dissipated in the telephone set and hence the performance of the portable telephone set in terms of sustainable duration of call is heavily dependent on the efficiency of signal transmission of the transmission power amplifying section. The efficiency of signal transmission of the transmission power amplifying section of the portable telephone set is expressed by the ratio of high frequency output power/input DC power. The efficiency is determined by the impedance characteristics of the matching circuits of the amplifiers. However, since the impedance characteristics of the matching circuits vary depending on the output power and the transmitter frequency, the impedance of the matching circuits has to be selected according to the transmitter frequency and the output power to be used for the portable telephone set.
In a prior art transmitter for mobile communication, the matching circuits of the amplifiers have a fixed impedance that is selected to realize a maximum efficiency for a maximum power output in the frequency band to be used with the portable telephone set in order to amplify a high frequency wave highly efficiently. This will be explained later in detail.
In the prior art transmitter, however, since the amplifiers are adapted to be matched by means of input/output matching circuits, the transmission power amplifying section of the transmitter is accompanied by the problem of a reduced efficiency at a low output level.
JP-A-57-60739 discloses a transmitter adapted to automatically select an output matching circuit that can minimize the efficiency out of a plurality of prearranged output matching circuits in order to improve the efficiency for low output operation. This transmitter is formed by a controller for issuing a switch signal for selecting an output matching circuit and an optimal value determining unit for selecting an optimal matching circuit and transmitting a signal to a matching switch circuit in addition to the components of the conventional portable telephone system.
However, with the transmitter of JP-A-57-60739, the amplifiers (transistors) of the transmitter are not modified if an optimal output matching circuit is selected as a function of the output power level. In other words, no improvement can be expected for the efficiency of the transmitter in the operating zone of 10 to 30 dB below the maximum output level if it is used for a portable telephone system. This is because, while large transistors having a large gate width in the case of FETs and having a size corresponding to the number of emitters in the case of bipolar transistors have to be used to accommodate the predetermined maximum output power, the transistors show a fall in the efficiency of amplification when they are driven in the operating zone of 10 to 30 dB below the maximum output level if compared with transistors having an optimal size. The reasons for this are as follows:
(1) The large transistors have a low impedance and hence cannot be matched perfectly. PA1 (2) The matching circuit itself that is matched with such a low impedance can show a high passing loss. PA1 (3) Generally, a large transistor is constructed by a plurality of unit cells coupled in parallel and the potential of the cells cannot be exploited fully due to the variances in the performance of the cells and the coupling loss of the cells. PA1 (4) A large transistor shows a low gain when compared with an element having a size optimally adapted to a low output level.
Thus, it will be obvious that the efficiency of operation of the transistors having a size optimally adapted to a low output level is by far higher than the efficiency of operation of the transistors having a larger size particularly in the operating zone around the maximum output level.
The ratio of the maximum transmission output and the minimum transmission output is referred to as output dynamic range, which is about 30 dB for conventional portable telephone sets. However, an output dynamic range of 60 to 70 dB is required for portable telephone sets to be used for a code division multiple access (CDMA) system that has become commercially available recently. Thus, it is particularly necessary for the transmission power amplifying section of a portable telephone set having such a wide dynamic range to show an improved efficiency.
However, with the transmitter of JP-A-57-60739, the transistor arranged in the last stage has a predetermined size and hence shows an efficiency of less than 10% for a low output level of about +10 to -10 dBm as in the case of the above described amplifiers if an optimal output matching circuit is selected out of a plurality of matching circuits as a function of the output level. Thus, the technique disclosed in JP-A-57-60739 is not effective for achieving a wide output range. It should be noted here that JP-A-57-60739 describes only a transmitter with 0.5 W (+27 dBm) for a low output level and 2 W (+33 dBm) for the maximum output level as a specific example.
Additionally, a transmitter as described in JP-A-57-60739 is constructed by PIN diodes as means for selecting an output matching circuit, which has to be constantly biased so that all the output matching circuits may be switched sequentially. Then, an electric current of several mA flows through the biased PIN diodes. Since a pair of PIN diodes are required for each of the output matching circuits, a total of 2.times.n PIN diodes have to be used for "n" matching circuits. If a PIN diode consumes power at a rate of 1 mA, then the power dissipation of all the PIN diodes will be n.times.2 mA regardless of the output power of the transmitter. This raises a serious problem for a portable telephone set that is required to be of an power saving type.